1. Field of the Invention
This invention relates to a purely chemical method for forming a layer of insoluble sulfides on semiconductor devices to passivate their surfaces and more particularly to a method of forming a layer of insoluble sulfides on a mercury cadmium telluride (Hg.sub.1-x Cd.sub.x Te) device surface in order to passivate the device surface.
2. Description Of The Prior Art
The description of the prior art that follows references processes involving Hg.sub.1-x Cd.sub.x Te. It is to be appreciated that the prior art processes can be practiced in connection with substances other than Hg.sub.1-x Cd.sub.x Te. More particularly, the method contemplated by the instant invention can be used with any semiconductor whose surface atoms will form stable insoluble sulfides. Accordingly, for the purposes of the description of the prior art, the utilization of methods with respect to Hg.sub.1-x Cd.sub.x Te will be examined as a specific example. It is against this back-drop of prior art examples that the advance contemplated by the present invention can be better appreciated.
Hg.sub.1-x Cd.sub.x Te is typically used in the manufacture of infrared (IR) detectors. Depending upon its composition or the value of x in its compositional formula it will operate over a wide range of wavelength regions. The wavelength of use depends on both the Hg.sub.1-x Cd.sub.x Te composition and the temperature at which it is used.
Surface properties have long been known to dominate many of the characteristics of pn semiconductor junctions of small geometry in Hg.sub.1-x Cd.sub.x Te and in other semiconductors. It is therefore important to have a suitable passivation process for this material in order to enhance and preserve the best detector properties. Passivation stabilizes the surface properties of a device.
Sulfidization as a passivation for semiconductors is a possible choice if the chemical literature indicates that a sulfide of one (or more) of the constituent elements is known form a stable sulfide. If the oxide of the constituent presents to the semiconductor device either chemical or electrical difficulties as a passivating agent (either during fabrication or in finished form) then the sulfide may be a useful alternative. Furthermore, if the sulfide is known to be insoluble, then the present chemical technique may be a viable passivation solution.
Heretofore, there have been at least two general types of passivation used for Hg.sub.1-x Cd.sub.x Te or other semiconductors. The first has been the growth of compounds on semiconductor surfaces using reactions which form the passivation film from the surface atoms of the semiconductor. The second type has been the deposition of foreign materials onto the surface of the semiconductor in order to generate this passivation.
In the first category there have been various insulators and semiconductors deposited onto the surface of the semiconductor. When the semiconductor is Hg.sub.1-x Cd.sub.x Te, these materials include cadmium telluride, silicon dioxide, and other highly resistive materials. In the second category the principal growth has been of films of oxides including native oxides that grow naturally on the surfaces of any semiconductor material exposed to air or moisture or water solutions, as well as oxides generated anodically or thermally or in plasmas.
It is well known in the art that native oxides on Hg.sub.1-x Cd.sub.x Te and many other semiconductors possess positive fixed charges which can invert the surfaces of p-type material. Therefore, even though native oxides often form high quality surface passivation for n-type material, they are inadequate for devices on p-type material. The latter material is normally used for photovoltaic infrared detectors.
In the second category, layers utilizing thin film deposition techniques have included materials such as evaporated zinc sulfide as well as silicon dioxide photochemically deposited at low temperatures. Deposited zinc sulfide tends to form moderately good interfaces on freshly etched Hg.sub.1-x Cd.sub.x Te or a few other semiconductor surfaces; however, inconsistent interfaces are often obtained on surfaces which have been exposed to chemicals during the processing cycle. In addition, excessive low frequency 1/f noise is often measured in these latter devices. This type of noise increases with reverse bias and is detrimental for the proper functioning of photovoltaic devices. Low temperature chemical vapor-deposited (CVD) silicon dioxide (SiO.sub.2), often photochemically assisted in growth, exhibits somewhat improved interface properties which may be used in conjunction with heterojunction detectors. However, SiO.sub.2 is absorbent above approximately 7 microns and is thus not suitable as coating for front illuminated 8 to 12 micron diodes. In addition, there are some adherence problems associated with deposited SiO.sub.2. The best results using SiO.sub.2 have been obtained if a few layers of natural or native oxide are present on the Hg.sub.1-x Cd.sub.x Te or other semiconductor before the SiO.sub.2 deposition; and even an extremely thin film, less that 100 Angstroms of native oxide, appears to protect the crystal against damage and also appears to improve adherence. This historical background illustrates the fact that native films can be indispensable for appropriately terminating the lattice while creating the minimum perturbation to the crystal lattice at the surface. The art of passivating films has not been limited to native oxides. More particularly, Nemirovsky and her associates at the Microelectronics Research Center, Department of Electrical Engineering at the Israeli Institute of Technology, have disclosed work relating to anodically deposited sulfide films on Hg.sub.1-x Cd.sub.x Te. Their initial results were reported in Applied Physics Letters, Vol. 44, No. 4, Feb. 15, 1984, page 443. A later and more detailed paper appeared in the Journal of Applied Physics, Vol. 58, No. 10, Jul. 1, 1985, page 366. The group also disclosed additional data in a paper presented at the 1985 U.S. Workshop on the Physics and Chemistry of Mercury Cadmium Telluride, Oct. 10, 1985, at San Diego.
In general terms, prior art anodic sulfidization is accomplished utilizing nonaqueous solutions of ethylene glycol in combination with dissolved sodium sulfide to produce the sulfide ion. These solutions are strongly basic, having a pH near 12. A constant current density of 60 to 140 microamperes per square centimeter is typically used in the process as the applied potential. The films grown using this prior art process have a thickness normally between the range of 400 to 500 Angstroms and at times up to as high as 600 Angstroms. On top of the anodized sulfide film approximately 3000 Angstroms or 0.3 micron of zinc sulfide is deposited by evaporation techniques.
In her 1985 article, Nemirovsky states:
"The formation of native sulfide films may be extended to additional compound semiconductors. In particular, it has been successfully applied to the III-V narrow band-gap semiconductor InSb, where a native film of indium sulfide forms an excellent interface." PA1 The layers adhere well to the surface because they are formed thereof; PA1 The layers result in a low semiconductor interface charge density and low semiconductor recombination velocity; PA1 They are stable with respect to their influence on electrical properties, producing low surface leakage and high R.sub.o A values, which in turn result in a high performance and reduced noise of finished detector devices; and PA1 The layers are compatible with device fabrication processes and are mechanically and thermally stable.
Just as the anodic method of Nemirovsky may be extended to additional compound semiconductors, the nonanodic method of the present invention may also be extended to additional compound semiconductors. While, in general, the ratio of the constituent elements of the semiconductor will be much different from the ratio of the constituent elements of the sulfide film (as Nemirovsky points out, only the indium, and not the antimony, forms a sulfide film), the resulting sulfide film has many useful characteristics. Compound semiconductors, one or more of the elements of which forms stable sulfides, are well known in the art. The present invention discloses a method whereby native films of such sulfides on such semiconductors may be formed with a nonanodic process, to complement the anodic process of Nemirovsky et al.
There are some inherent disadvantages of the anodic process. Firstly, it is an electrochemical process or an anodization which takes place in a solution under an applied potential. Such a process requires the attachment of electrodes on the semiconductor substrate in order to have it act as one of the electrodes in solution. These electrodes must later be removed from the substrate. Not only do the electrodes act as sources of contamination, but also, the application and removal of the electrodes increases the likelihood of physically damaging the substrate as well as the semiconductor devices thereon.
Secondly, the anodic methods typically use films of thickness 400 to 600 Angstroms. Films of such, or increased, thickness, especially when prepared by anodization, often can result in layers which are structurally non-uniform and which inevitably are not free of stress or structural imperfections. The latter type defects are most clearly seen in the case of anodized aluminum oxide films wherein the structure in thicker films eventually leads to columnar growth (a structural nonuniformity) which is not desirable in good passivating layers. Columnar growth is associated with less perfect structures and often has less dense or even porous material properties.
Thirdly, the nonaqueous solutions of ethylene glycol in combination with dissolved sodium sulfide contain sodium ions which are difficult to remove from semiconductor surfaces because they adsorb strongly. Lastly, these solutions are strongly basic and thus tend to destructively attack the surfaces being treated.
In summary, although the anodization techniques do appear to yield improved photovoltaic devices because of their passivation properties, the method used is difficult to apply to semiconductor manufacturing technology in high volume and with high yield. Anodization processes offer possible contamination effects and nonuniformity, together with yield losses due to the difficulties in attaching/removing contacts to semiconductor structures or wafers. They involve solutions that are both strongly basic and contain contaminating sodium ions. Sodium ions are difficult to remove from semiconductor surfaces since they adsorb strongly. Basic solutions, especially strong alkaline concentrations of high pH, are able to destructively attack Hg.sub.1-x Cd.sub.x Te and other semiconductor surfaces (e.g., strong basic solutions can be used to destructively pit semiconductor surfaces to reveal imperfections).
From the foregoing, the need should be appreciated for a new and improved method for passivation of Hg.sub.1-x Cd.sub.x Te and other semiconductor surfaces. More particularly, a method for passivation of semiconductor surfaces creating a passivating film that has the following qualities is desirable:
Accordingly, a fuller understanding of the invention may be obtained by referring to the SUMMARY OF THE INVENTION, and the DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT, in addition to the scope of the invention defined by the claims in conjunction with the accompanying drawings.